The Experts below are selected from a list of 324 Experts worldwide ranked by ideXlab platform
Sebastian Diehl - One of the best experts on this subject based on the ideXlab platform.
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Temperature dependence of the type III Functional Response in Daphnia
2016Co-Authors: Wojciech Uszko, Sebastian DiehlAbstract:Consumer-resource interactions are the basic building blocks of every food web. In spite of being a central research theme of longstanding interest in ecology, the mechanisms governing the stability and persistence of consumer-resource interactions are still not entirely understood. In particular, theoretical predictions on consumer-resource stability along gradients of temperature and nutrient enrichment diverge widely and are sometimes in conflict with empirical results. In this thesis I address these issues from the angle of the Functional Response, which describes a consumer’s feeding rate as a function of resource density. Specifically, I explore mechanistic, nutrient-based consumer-resource interaction models with respect to the influence of feeding behavior (the shape of the Functional Response), environmental temperature, nutrient enrichment, and resource quality on consumer-resource stability and persistence. In order to parameterize these models I performed extensive laboratory experiments with pairs of freshwater pelagic algae and grazers of the genus Daphnia, which are widespread, ecologically important model organisms.I found a sigmoidal type III Functional Response in every studied Daphnia-algae species pair. The exact form of its shape is described by an exponent b which is determined by fitting Functional Response models to the experimental data. A high value of b can stabilize consumer-resource systems under the otherwise destabilizing influence of nutrient enrichment, as predicted by a novel stability criterion relating b to the consumer’s prey handling time, food conversion efficiency and mortality. Estimated parameter values and, consequently, stability predictions are sensitive to the method of parameter estimation, and I propose a new estimation procedure that minimizes parameter uncertainty. Because many consumers’ feeding rates depend on temperature, warming is expected to strongly affect food web stability. In Functional Response experiments over a broad temperature gradient, I found that the attack rate coefficient and the maximum ingestion rate of Daphnia are hump-shaped functions of temperature. Moreover, the Functional Response exponent increases with warming towards stronger type III Responses. Plugging these findings into a nutrient-based consumer-resource model, I found that predator persistence is a U-shaped function of temperature in nutrient enrichment-temperature space. Enrichment easily turns the system unstable when the consumer has a type II Response, whereas a type III Response opens up a large region of stability at intermediate, for the consumer optimal, temperatures. These findings reconcile seemingly conflicting results of earlier studies of temperature effects on consumer-resource dynamics, which can be mapped as special cases onto the enrichment-temperature space. I finally demonstrate the utility of three key model ingredients - temperature dependence of rate parameters, a mechanistic description of the dynamics of algal resources, and a type III Functional Response in Daphnia - by successfully implementing them in the description and explanation of phytoplankton-Daphnia dynamics in a mesocosm experiment exploring effects of warming on the spring succession of the plankton.
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Temperature dependence of the Functional Response
Ecology Letters, 2011Co-Authors: Goran Englund, Gunnar Öhlund, Catherine L. Hein, Sebastian DiehlAbstract:The Arrhenius equation has emerged as the favoured model for describing the temperature dependence of consumption in predator–prey models. To examine the relevance of this equation, we undertook a metaanalysis of published relationships between Functional Response parameters and temperature. We show that, when plotted in lin-log space, temperature dependence of both attack rate and maximal ingestion rate exhibits a hump-shaped relationship and not a linear one as predicted by the Arrhenius equation. The relationship remains significantly downward concave even when data from temperatures above the peak of the hump are discarded. Temperature dependence is stronger for attack rate than for maximal ingestion rate, but the thermal optima are not different. We conclude that the use of the Arrhenius equation to describe consumption in predator–prey models requires the assumption that temperatures above thermal optima are unimportant for population and community dynamics, an assumption that is untenable given the available data.
Herbert H. T. Prins - One of the best experts on this subject based on the ideXlab platform.
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Increased searching and handling effort in tall swards lead to a Type IV Functional Response in small grazing herbivores
Oecologia, 2011Co-Authors: Nicol Heuermann, Frank Langevelde, Sipke E. Wieren, Herbert H. T. PrinsAbstract:Understanding the Functional Response of species is important in comprehending the species’ population dynamics and the functioning of multi-species assemblages. A Type II Functional Response, where instantaneous intake rate increases asymptotically with sward biomass, is thought to be common in grazers. However, at tall, dense swards, food intake might decline due to mechanical limitations or if animals selectively forage on the most nutritious parts of a sward, leading to a Type IV Functional Response, especially for smaller herbivores. We tested the predictions that bite mass, cropping time, swallowing time and searching time increase, and bite rate decreases with increasing grass biomass for different-sized Canada geese ( Branta canadensis ) foraging on grass swards. Bite mass indeed showed an increasing asymptotic relationship with grass biomass. At high biomass, difficulties in handling long leaves and in locating bites were responsible for increasing cropping, swallowing, and searching times. Constant bite mass and decreasing bite rate caused the intake rate to decrease at high sward biomass after reaching an optimum, leading to a Type IV Functional Response. Grazer body mass affected maximum bite mass and intake rate, but did not change the shape of the Functional Response. As grass nutrient contents are usually highest in short swards, this Type IV Functional Response in geese leads to an intake rate that is maximised in these swards. The lower grass biomass at which intake rate was maximised allows resource partitioning between different-sized grazers. We argue that this Type IV Functional Response is of more importance than previously thought.
Goran Englund - One of the best experts on this subject based on the ideXlab platform.
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Temperature dependence of the Functional Response
Ecology Letters, 2011Co-Authors: Goran Englund, Gunnar Öhlund, Catherine L. Hein, Sebastian DiehlAbstract:The Arrhenius equation has emerged as the favoured model for describing the temperature dependence of consumption in predator–prey models. To examine the relevance of this equation, we undertook a metaanalysis of published relationships between Functional Response parameters and temperature. We show that, when plotted in lin-log space, temperature dependence of both attack rate and maximal ingestion rate exhibits a hump-shaped relationship and not a linear one as predicted by the Arrhenius equation. The relationship remains significantly downward concave even when data from temperatures above the peak of the hump are discarded. Temperature dependence is stronger for attack rate than for maximal ingestion rate, but the thermal optima are not different. We conclude that the use of the Arrhenius equation to describe consumption in predator–prey models requires the assumption that temperatures above thermal optima are unimportant for population and community dynamics, an assumption that is untenable given the available data.
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The Functional Response of a predatory plant preying on swarming zooplankton
Oikos, 2001Co-Authors: Goran Englund, Sabine HarmsAbstract:In a laboratory study, we determined the Functional Response of the carnivorous aquatic plant Utricularia vulgaris feeding on Polyphemus pediculus, a cladoceran zooplankton that forms swarms. The number of prey eaten increased linearly with prey density up to a density of 35 prey per 125 ml and decreased slightly above this density. Independent estimates of handling time showed that the number eaten was not limited by handling. Thus, we hypothesized that the Functional Response levelled off because attack rate decreased with increasing density. Direct observations of the predation act at high and low prey densities showed that prey per capita mortality rate was markedly lower at high densities. An analysis of the components of the predation cycle showed that encounter rate and attack probability but not capture success decreased with increasing prey density. We, then, studied the degree of aggregation and the movement behaviour of Polyphemus. The tendency to form swarms increased with density and this was associated with reduced swimming speed and swimming along a more tortuous path. Presence of Utricularia leaves did not influence the spatial distribution and swimming behaviour of Polyphemus. We concluded that the unusual shape of the Functional Response was due to density dependent prey mortality rates that resulted from a density dependent tendency to form swarms. We, therefore, suggested a modification of Holling's type II Functional Response model that included density dependent attack rate and this model fitted data significantly better than the original model.
Toshinori Okuyama - One of the best experts on this subject based on the ideXlab platform.
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On selection of Functional Response models: Holling’s models and more
BioControl, 2013Co-Authors: Toshinori OkuyamaAbstract:Model selection is a common and established research method. Statistically rigorous model selection methods are used in a variety of research fields. In contrast, studies that characterize Functional Response models commonly use a model selection method that is specific to Functional Response studies. The specific method aims to distinguish between Holling’s type II and type III Functional Response models. This paper discusses problems associated with the specific method and suggests that it would be better to use general model selection methods that allow to consider a variety of models.
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A likelihood approach for Functional Response models
Biological Control, 2012Co-Authors: Toshinori OkuyamaAbstract:Abstract Functional Response is an important determinant of community dynamics, and thus empirical methods for characterizing Functional Responses are as important in understanding ecological processes. The most commonly used method is based on the sum of squares, and the maximum likelihood method is rarely used. When the likelihood method is used, potentially inappropriate probability distributions such as binomial distributions are typically assumed for the number of prey eaten in experiments. In this study, I present a likelihood approach in which the probability distributions are generated by mechanistic understanding of predation processes using Monte Carlo simulations. An example is given on the Holling type II Functional Response model, but the method is flexible and allows characterization of a wide variety of Functional Response models. In the example, the likelihood method consistently resulted in superior estimates than the least squares method.
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Solutions for Functional Response experiments
Acta Oecologica, 2011Co-Authors: Toshinori Okuyama, Robert L. RuyleAbstract:Abstract Functional Response is an important factor in ecological processes. Many empirical studies have characterized Functional Responses based on model selection procedures. However, relatively few models have been considered in such studies. This may be partly because methods for characterizing general Functional Response models are not well known. In this study, we present the derivation of explicit and implicit solutions for a wide class of Functional Response models which includes predator-dependent models and flexible trait models. These solutions can facilitate the future characterizations of Functional Responses in empirical studies.
Ken Norris - One of the best experts on this subject based on the ideXlab platform.
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Vigilance and the Functional Response of granivorous foragers.
Functional Ecology, 2010Co-Authors: David Baker, James M Bullock, Richard A. Stillman, Barbara Smith, Ken NorrisAbstract:1. Functional Response models that predict the relationship between feeding rate and food density often include only two behavioural parameters, handling time and searching rate. However, vigilance can occupy a large proportion of foraging time and, consequently, may affect the Functional Response. Previous Functional Response models of granivorous birds showed no effect of vigilance on predicted feeding rates; these models assumed that all of handling time is compatible with vigilance and, therefore, overestimated the potential time for cost free vigilance to occur. 2. Here we have derived a new Functional Response model that incorporates the proportion of time spent vigilant (v) and the proportion of handling time that is compatible with vigilance (p). This model allows for the relationship between vigilance and handling to vary from completely compatible to mutually exclusive, and degrees in between. 3. To determine whether vigilance can affect the Functional Response of a granivorous bird, grey partridge Perdix perdix L, we measured the Functional Response and associated behavioural parameters, and used the behavioural estimates to parameterize the model. Any deviation from the feeding rates predicted using a model without vigilance indicates that vigilance is affecting the predicted Functional Response. 4. We found that vigilance only affected the predicted Functional Response at very low food densities ( 50% of their foraging time vigilant, and found that even with a high value of p vigilance reduced feeding rates at higher seed densities. 5. This study shows that vigilance can affect the feeding rate of a granivorous bird when either the proportion of time spent vigilant is high or the proportion of compatible handling time is low. This may affect larger scale ecological processes, i.e. spatial distribution of foragers and patterns of resource depletion, as individuals try to mitigate the effects of vigilance by maximizing their feeding rate whilst minimizing their predation risk.
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Searching efficiency and the Functional Response of a pause‐travel forager
Functional Ecology, 2007Co-Authors: Alison E. Poole, Richard A. Stillman, H. K. Watson, Ken NorrisAbstract:1. The feeding rates of many predators and parasitoids exhibit type II Functional Responses, with a decelerating rate of increase to reach an asymptotic value as the density of their prey or hosts increases. Holling's disc equation describes such relationships and predicts that the asymptotic feeding rate at high prey densities is set by handling time, while the rate at which feeding rate increases with increased prey density is determined by searching efficiency. Searching efficiency and handling time are also parameters in other models which describe the Functional Response. Models which incorporate Functional Responses in order to make predictions of the effects of food shortage thus rely upon a clear understanding and accurate quantification of searching efficiency and handling time. 2. Blackbird Turdus merula exhibit a type II Functional Response and use pause-travel foraging, a foraging technique in which animals search for prey while stationary and then move to capture prey. Pause-travel foraging allows accurate direct measurement of feeding rate and both searching efficiency and handling time. We use Blackbirds as a model species to: (i) compare observed measures of both searching efficiency and handling time with those estimated by statistically fitting the disc equation to the observed Functional Response; and (ii) investigate alternative measures of searching efficiency derived by the established method where search area is assumed to be circular and a new method that we propose where it is not. 3. We find that the disc equation can adequately explain the Functional Response of blackbirds feeding on artificial prey. However, this depends critically upon how searching efficiency is measured. Two variations on the previous method of measuring search area (a component of searching efficiency) overestimated searching efficiency, and hence predicted feeding rates higher than those observed. Two variations of our alternative approach produced lower estimates of searching efficiency, closer to that estimated by fitting the disc equation, and hence more accurately predicted feeding rate. Our study shows the limitations of the previous method of measuring searching efficiency, and describes a new method for measuring searching efficiency more accurately.